The present invention relates generally to touchpad devices, and more particularly to capacitive sensing touchpad devices.
A wide variety of touchpad or touch sensing devices have been developed. Resistive membrane position sensors have been used for some time. Such touchpads have been used as keyboards, position indicators, etc. Other types of touch sensing devices include resistive tablets, surface acoustic wave devices, touch sensors based on strain gauges or pressure sensors, and optical sensors.
Capacitive touchpads are often used to detect a finger. A typical capacitive touchpad will have a matrix of row and column traces separated by an insulating or dielectric layer from a ground plane. When a finger is brought into proximity with the matrix, the finger acts as a conductor which intercepts the electromagnetic radiation when the individual traces are driven. Thus, it will capacitively couple, changing either the capacitance between the matrix and ground, or the transcapacitance between row and column electrodes. This variation in capacitance can be measured as a change in voltage. By sequentially driving a current to all of the traces in rapid succession, and measuring the resulting voltages on them, the location of the finger can be determined.
Typically, a different system has to be used for a stylus. A stylus may be of a plastic or other material which is non-conductive, and thus will not provide the same capacitive effect as a finger. One method used is to simulate a finger by providing an active stylus, which has an electrical circuit in it for generating signals which will interact with the traces, and produce a similar effect to that of a finger.
It would be desirable to have a single touchpad technology which can detect both a finger and an inactive stylus.